1. Field of the Invention
The invention relates to modulation systems and more particularly to modulation arrangements in which the carrier is suppressed.
2. Background of the Invention
In many types of modulated transmission systems, a carrier modulated by an information signal is applied to a medium for transmission. The information signal may be analog or digital and of electrical or optical form. The power in the modulated signal is distributed among the carrier and modulation components. In satellite and other communication systems where the spectrum utilized for communication is crowded, the transmission power of individual channels is restricted to avoid inter-channel interference. To reduce the channel power without impairing information transmission, it is well known, to suppress the carrier component of the transmission signal. Such carrier suppression is important in making wideband frequency allocations to existing narrowband services, in spread spectrum systems where it is difficult to meet regulated spectral density requirements and in a Weaver Demodulator in which a signal is processed to produce an output in the form of an upper sideband.
Carrier suppression has been performed through the use of filters, by precise adjustment of modulator parameters and by balanced type modulators such as described in U.S. Pat. No. 5,450,044 issued to Timothy P. Hulick on Sep. 12, 1995, or in U.S. Pat. No. 4,748,641 issued to Mark J. Dapper on May 31, 1988. In the Dapper patent, a binary phase shift keying type BPSK modulator is disclosed in which the I (in-phase) modulating component of a BPSK signal is mixed with an RF carrier source in one mixer, the Q (quadrature-phase) modulating component of the BPSK signal is mixed with the RF carrier phase shifted by 90 degrees in another mixer and the outputs of the mixers are summed in a summing network. The summing network output then provides a suppressed carrier signal according to the matching of the operating characteristics of the mixers and the balance of modulating signal amplitudes and phases. The degree of carrier suppression in filtered and balanced modulator systems, however, relies on the precise adjustment of the operating characteristics in the transmitter to obtain cancellation of the carrier in the summing network. Such precise adjustment, however, is difficult to achieve and maintain under changing ambient conditions such as temperature, aging, and radiation effects.
In another carrier suppression arrangement for a QAM transmitter disclosed in U.S. Pat. No. 6,687,311 issued to Qin Zhang on Feb. 3, 2004, a monitoring device monitors phase and amplitude errors in an output RF signal with QAM components and provides a feedback signal to an equalizer. The equalizer produces an analog equalizing signal that equalizes the phases and amplitudes of the I and Q components of the QAM signal. Such feedback arrangements are effective to provide carrier suppression without precise modulator adjustments but add significant complexity and cost. The feedback arrangements also require settling time from turn-on until a satisfactory level of carrier suppression level is achieved and are ineffective for burst-communication systems.
FIG. 1 illustrates the operation of a feedback arrangement to stabilize carrier suppression. In FIG. 1, there is a modulator 105, a carrier level detector 110, a carrier null control 115, and a carrier null adjuster 120. A portion of the output of the modulator 105 is supplied to the carrier level detector 110 and the carrier null control 115 provides a carrier null signal responsive to the detected carrier level. The carrier null signal from the carrier null control 115 is combined with a manual adjustment signal in the carrier null adjuster 120. The output of the carrier null adjuster is combined with the input signal through combiner 101 and the combined output is applied as the modulating signal input to the modulator 105. The modulator 105 receives the carrier and an adjusted modulating signal derived from an input signal and the carrier null adjuster 120. The implementation of a feedback arrangement for carrier suppression, however, is subject to manual adjustment and requires added equipment and a significant setting time from turn-on. In satellite and other communication systems where space, weight, and maintenance access are important, the added feedback type carrier suppression equipment is undesirable. Accordingly, it is a problem to provide adequate carrier suppression without requiring either precise adjustment of transmitter parameters or the addition of feedback type carrier suppression equipment.
U.S. Pat. No. 4,447,907 issued to John E. Bjornholt et al., May 8, 1984, discloses a multiple mixer spread spectrum modulation arrangement in which there are plural serially coupled bi-phase modulators and a set of code generators which generate plural binary code signals. The plural binary code signals are phased to have equal relative phase angles and serially modulate an input carrier signal. Each modulator spreads the carrier signal applied thereto. The signal power of the modulated carrier after the serially coupled bi-phase modulators is spread over a range of frequencies determined by the binary coded modulating signals. As a result, the Bjornholt et al. arrangement provides a spread carrier signal that resembles noise for use in a spread spectrum communication rather than a carrier suppressed information signal. Accordingly, the Bjornholt et al. arrangement is suitable only for wideband transmission systems that require or utilize pseudo-random noise spectral spreading signals.